1. Field of the Invention
This invention relates to totalizer units for belt scales, flowmeters and other devices for measuring the bulk, mass or weight of material flowing or being conveyed past a fixed point at which the instantaneous loading on a suitable transducer or transducers is continuously sensed.
2. Review of the Art
Many such arrangements are already known. In general, a transducer or group of transducers is arranged to generate an electrical signal proportional to the instantaneous loading applied to an element which reacts to the flow of material, and this signal is multiplied by the speed of flow and integrated in order to produce an output corresponding to the quantity of material which has passed the element. In flowmeters, the apparatus may be arranged so that the speed of flow is constant, while in belt scales the speed of the belt is measured by a suitable sensor such as a pulse generating tachometer.
Processing of the transducer and tachometer signals is rendered more complex by the necessity of providing for zeroing of the transducer output. The zero load output of the transducer may shift, for example, due to changes in transducer characteristics because of material adhering to the conveyor belt in a belt scale or the sensor plate in a flowmeter, because of wear of the belt in a belt scale, or because of temperature or humidity variations. Any such zeroing in a belt scale must allow for the tare of the belt, which varies not only as the belt wears, but from point to point on the belt itself. The belt will usually be heavier in the immediate vicinity of a splice in the belt as well as having other irregularities. For this reason, zeroing of control and display units for belt scales is usually performed by monitoring output over one or more full revolutions of the belt through the system so as to find a setting which will produce an overall integrated output which is zero.
In apparatus which requires frequent zeroing, it is desirable to have this occur automatically without operator intervention. While various manufacturers have developed techniques for carrying out such zeroing automatically, automation of this feature presents problems which have prevented its widespread adoption.
Typically autozeroing is achieved by carrying out a zeroing cycle during periods when there is no flow through the apparatus. A problem with belt scales and to a lesser extent with flowmeters is that it is difficult to distinguish between a low rate of flow and no flow. A belt scale operating in conjunction with a belt having heavy and light sections will produce a varying transducer output even under no flow conditions.
Belt scale systems are also usually set up so that outputs corresponding to very low rates of flow, below a certain threshold, are disregarded. Any such threshold setting must be disabled prior to zeroing, since otherwise it will act so as to shift the zero setting. An autozeroing operation must also be scheduled so as not to interfere with normal operation of the apparatus when a load is present. Unfortunately, it is not possible for the apparatus to predict when a gap in the flow of material will occur which is long enough for the autozeroing function to be completed. If flow recommences while the zeroing function is in progress, both the function itself will be aborted, and material will pass through the system without being weighed. If a substantial lump of material temporarily sticks on a belt or flowmeter element, it may produce a temporary false zero which will cease to be valid as soon as the lump shifts.